High-voltage fuse



Oet. 6, 1970 E. E. MCcoNNELL HIGH-VOLTAGE FUSE Filed April 25, 1969 FIG nited States Patent() U.S. Cl. 337--162 3 Claims ABSTRACT OF THE DISCLOSURE A high voltage fuse having an outer tubular casing and three inner casings shorter than the outer casing arranged parallel to, and inside of, the outer casing, and closed on both ends by cylindrical ferrules. Each of the three inner casings is lled with a non-gas-evolving pulverulent arcquenching iiller, and its ferrules are conductively interconnected by fusible elements designed to fuse sequentially when subjected to the same short-circuit current. The three aforementioned inner casings include a pair of inner casings each arranged relatively close to the one of the axially outer ends of the outer casing and an additional inner casing arranged relatively close to the center of the axially outer casing. The ferrules on the axially inner ends of each of said pair of casings overlap the ferrules on the axially outer ends of said additional inner casing and overlapping ferrules are conductively interconnected.

`Baolnground of invention The tubular casings of high-voltage fuses are normally relatively thin and relatively long. They tend to become thinner as the magnitude of the current the particular fuse is required to carry normally decreases, and longer the higher the voltage of the system in which the particular `fuse is intended to be applied. As a result, for certain current and voltage ratings the diameters of the castings become critically small, and their length excessive. This condition can be remedied to some extent by imparting a zig-zag configuration to the fusible element, its constituent sections enclosing acute angles. This results in a fuse of relatively reduced length and relatively increased diameter. In such designs there is, however, a danger of short-circuiting of the arc resulting from melting and vaporization of the zig-zag shaped fusible element at the points of that arc corresponding to the points where the fusible element sharply reverses it direction, or forms loops at acute angles. Again this conditionk can be remedied by separating the loop-forming sections of the fuse link by means of insulating 'barriers which may be tubular and prevent the aforementioned drawback of short-circuiting portions of the arc which results from melting and vaporization of the fusible element.

Even if such insulating barriers are provided, the zigzag geometry of the fusible elements of high-voltage fuses of the kind under consideration is not conducive to satisfactory performance characteristics. One of the reasons for this limitation of high-voltage fuses vvhose fusible elements have a zig-zag shape involving small or acute angles is due to the fact that but a small increase in the aggregate length of the fusible element over and above the length of the casing can be achieved. In fuses of the above description the minimum spacing of each loop from the terminal element of the fuse immediately adjacent thereto depends upon the dielectric properties of the material which is present in this space. If this space is lled with a material that has relatively poor dielectric properties, the aforementioned minimum spacing must be relatively large, and consequently the aggregate length of the fusible element cannot be significantly longer than the length of the casing. If the products of arcing result- Patented Oct. 6, 1970 "ice ing from melting and vaporization of the fusible element ow, or to diffuse, into the aforementioned material, its dielectric properties are bound to be adversely affected. Under such conditions it is hardly worthwhile to incur the additional cost involved in manufacturing fuses whose fusible elements zig-zag at acute angles from one terminal element to the other.

The successful operation of high-voltage fuses depends upon the generation of relatively high pressures within the casing of the fuse fwhen the latter interrupts a major fault current. If the zig-zag :geometry results in an increased volume, not only a shortened length of the casing of the fuse, the pressure build-up during the process of interruption tends to be insuicient to satisfy high performance standards.

This invention avoids the drawback of the above referred-to prior art high-voltage fuses featuring a zig-zag geometry of their fusible elements whose length exceeds that of the casings and whose loop-shaped portions are separated by insulating barrier precluding short-circuiting of arc loops and consequent reduction of the arc voltage.

Summary of invention A high-voltage fuse embodying this invention includes an outer casing of electric insulating material and a pair of terminal elements closing the outer ends of said casing. The fuse further includes a pair of inner tubular casings of electric insulating material having a shorter length than said outer casing and arranged inside of, and in parallel relation to, said outer casing. Each of said pair of inner casings is filled with non-gas-evolving pulverulent arc-quenching ller and closed on both ends thereof by external cylindrical ferrules. The axially outer ferrule of each of said pair of inner casings is conductively related to one of said pair of terminal elements immediately adjacent thereto. The axially inner ferrules of said pair of inner casings are spaced in a direction longitudinally of vsaid outer casing. A fuse embodying this invention further includes an additional casing of electric insulating material having a length shorter than the length of said outer casing Ibut in excess of the spacing between the axially inner ferrules of each of said pair of inner casings. The aforementioned additional casing is arranged inside of and parallel to said outer casing. The additional inner casing is lilled with a non-gas-evolving pulverulent arc-quenching filler and closed on both ends thereof by external cylindrical ferrules. Each ferrule of said additional inner casing overlaps an axially inner ferrule on one of said pair of inner casings and overlapping ferrules are conductively interconnected. The space inside of said outer casing not occupied by said pair of inner casings and said additional inner casing is filled with an electrically insulating heat transfer medium. A plurality of separate fusible elements embedded in said arc-quenching iller in said pair of inner casings and in said arc-quenching tller in said additional casing conductively interconnect the ferrules on said pair of inner casings and conductively interconnect the ferrules on said additional inner casing. The fusible elements contained in different casings have points of minimum crosssectional area of diiferent sizes to melt and vaporize sequentially in response to a short-circuit current.

BRIEF DESCRIPTION OF DRAWINGS FIG. l shows a composite high-voltage fuse embodying this invention partly in longitudinal section and partly in front elevation;

FIG. 2 is a section along II-II of FIG. l;

FIG. 3 is an isometric view of the structure of FIG. l with some portions of the outer casing being broken away to show the interior thereof;

FIG. 4 is a longitudinal section of a .fuse forming a subassembly of the structure of FIG. l;

FIG. is a longitudinal section of another fuse forming `a subassembly of the structure of FIG. l;

FIG. 6 is a section along VI-VI of FIG. 4; and

FIG. 7 is a section along VII-VII of FIG. 5.

Description of preferred embodiment of the invention The outer casing 1 of electric insulating material is provided with a pair of terminal elements 2 closing the outer ends of casing 1. Terminal elements 2 are in the form of caps mounted on casing 1 and held in position by drive screws 3 projecting transversely through caps 2 and casing 1. One of terminal elements 2 is provided with an externally screw-threaded terminal stud 4, and the other terminal element 2 is provided with an internally screw-threaded terminal stud 5. A pair of tubular casings 6a, 6b of electric insulating material having a shorter length than casing 1 are arranged inside of casing 1 in parallel relation to the latter. FIG. 5 shows the details of the parts associated with casings 6a, 6b. Both casings 6a, 6b are filled with a non-gas-evolving pulverulent arcquenching ller 8, e.g. quartz sand, and closed on both ends thereof by cylindrical ferrules 7. The axially outer ferrules 7 of each of said pair of casings 6a, 6b are conductively related to terminals 2. To this end terminals 2 may be provided with perforations 2a for introducing a drop of solder to the fine gaps formed between the inner surfaces of terminals 2 and the axially outer surfaces of ferrules 7. The axially inner ferrules 7 on each of said pair of casings 6a, 6b are spaced in a direction longitudinally of outer casing 1. In addition to inner casings 6a, 6b outer casing 1 houses the inner tubular casing 6c of inslating material. Casing 6c is shorter than casing 1- but its length A exceeds the spacing B between the axially inner ferrules 7 on casings 6a, 6b. Casing 6c is arranged inside 0f casing 1 in parallel relation to it. Casing 6c is filled with a non-gas-evolving pulveiulent arc-quenching filler 9 which may be quartz sand. If quartz sand is selected as arc-quenching filler in casings 6a, 6b and in casing 60, it may be desirable to use a different spectrum of particle sizes in casing 6c than in casings 6a and 11. Both ends of casing 6c are closed `by external cylindrical ferrules 10. Each of ferrules 10 is arranged to overlap and to engage an axially inner ferrule 7 on one of the pair of casings 6a, 6b along a common generatrix thereof. Engaging pairs of ferrules 7, 10 are preferably connected by solder joints (not shown) extending along the common generatrices thereof. The space inside of casing 1 not occupied by inner casings 6a, 6b, 6c is filled with an electrically insulating heat transfer medium 17. Since this medium does not need to have any arc-quenching ability another medium than a conventional arc-quenching medium may be used to transfer heat generated in casings 6a, 6b, 6c to the casing 1, from where it is dissipated to atmosphere. Such arc-quenching media which are relatively good thermal insulators, e.g. gypsum, are unsuitable for the transfer of heat from inner -casings 6a, 6b, 6c to the outer casing 1 and to atmosphere, but medium 17 may be fine quartz sand. Casings 6a, 6b each house a ribbon fuse link embedded in arc-quenching filler 8 and conductively interconnecting the ferrules 7 on the ends of casings 6a, 6b. The ends of ribbon fuse link 11 project through the end surfaces of ferrules 7 and are conductively connected to ferrules 7 by means of solder joints 12. Ribbon fuse links 11 project through sub-housings 13 inside of -casings 6a, .6b which sub-housings preclude access of filler 8 to a limited length of fuse links 11, thus establishing voids around links 11 and raising the temperature of links 11 at the points Where they are surrounded by a gaseous medium rather than by arc-quenching filler 8. Thus relatively hot zones are established along fuse links 11 which are ofr' center both in regard to outer casing 1 and in regard to inner casings 6a, 6b. Fuse links 11 are provided with an overlay 14 of a link-severing metal having a lower melting point than the base metal of ribbon links 11. The former is preferably tin and the latter preferably silver. When overlays 14 melt as a result of a protracted overload current of inadmissible duration links 11 are severed by metal diffusion phenomena and arcs are formed inside of subhousings 13. Since these arcs are separated from arc-quenching iiller 8 by voids, the danger of formation of fulgurites in case of overload current interruptions is effectively avoided. The fact that the position of sub-housings 13 as well a-s the number of heatgenerating points of reduced cross-sectional area of perforated ribbon-links llwhich are situated inside of sub-housing 13 are optional makes it readily possible to exert a far-reaching control of the geometry of the timecurrent-curve of a composite fuse according to this invention.

The pulverulent arc-quenching filler 9 in casing 6c surrounds a fusible element 15 in wire form. The uniform cross-sectional area of fusible element 15 is less than the minimum cro-ss-sectional area of ribbon fuse links 11. Fusible element 15 in wire form conductively interconnects the ferrules 10. Solder joints 16 on the outside of ferrules 10 minimize the voltage drop between wire 15 and ferrules 10.

The current path of the composite fuse extends from one terminal element 2 through one of ferrules 7 and of ribbon 11 in casing 6a to the axially inner ferrule 7 on casing 6a, and then through one of the ferrules 10 on casing 6c through wire 15 to the other ferrule 10` on casing 6c, 'and then through the axially inner ferrule 7 on casing 6b and through ribbon fuse link 11 in casing 6b and through the axially outer ferrule 7 on casing 6b to the other terminal element on casing 1.

On occurrence of a major faulty current, or shirt-circuit current, the Iwire 15 melts and vaporizes before the ribbon fuse link 11 -in casings 6a, 6b melts and vaporizes, the melting and vaporizing t2 value of wire 15 being less than the melting and vaporzing i2-t values of ribbon links 11. This makes it possible to effectively control the Ivoltage surge incident to blowing of the fuse at major fault currents, or short-circuit currents. Since the spaces occupied by fusible elements 15 and 11 are physically separated it is readily possible to provide diiferent arcquenching fillers in casings 6a, 6b and 6c. It may, for instance, be desirable to surround wire 15 with quartz sand of finer particle size than the quartz sand surrounding ribbon fuse links 11.

The composite fuse according to this invention is in effect a modular system, or a system of building blocks since it makes it readily possible to combine three different types of fuse sub-units and thus to obtain a wide variety of ratings and performance characteristics with a minimum of differing parts.

As mentioned above the registering ferrules 7, 10 on casings 6a, 6b, 6c are preferably in physical engagement along a common generatrix thereof and joint by solder along the aforementioned generatirx. If desired, the end surfaces of the aforementioned ferrules 7, 10 may also be conductively interconnected by metal straps 17, as shown in FIG. 34

I claim as my invention:

1. A high-voltage fuse including (a) an outer tubular casing of electric insulating material;

(b) a pair of terminal elements closing the outer ends of said casing;

(c) a pair of inner tubular casings of electric insulating material having a shorter length than said outer casing arranged inside of and in parallel relation to said outer casing, each of said pair of inner casings being filled with a non-gas-evolving pulverulent arcquenching ller and closed on both ends thereof by external cylindrical ferrules, the axially outer ferrule on each of said pair of inner casings being conductively related to one of said pair of terminal elements immediately adjacent thereto and the axially inner ferrule on each of 'said pair of inner casings being spaced in a direction longitudinally of said outer minimum cross-sectional area of' different sizes to casing; melt and vaporize sequentially in response to a short- (d) an additional inner tubular casing of electric incircuit current.

sulating material having a length shorter than the 2. A high-voltage fuse as specified in claim 1 wherein length of said outer casing and in excess of the 5 said axially inner ferrule on each of said pair of inner` spacing between said axially inner ferrule on each of casings engages one of said ferrules on said additional said pair of inner casings arranged inside of and casing along a common generatrix thereof. parallel to said outer casing, said additional inner 3. A high-voltage fuse as specified in claim `1 wherein casing being filled withanon-gas-evolving pulverulent said ferrules on each of said pair of inner casings are arc-quenching ller and closed on both ends thereof 10 conductively interconnected by ribbon fusible elements by external cylindrical ferrules, each ferrule on said and wherein said ferrules of said additional inner casing additional inner casing overlapping and being conare conductively interconnected by a fusible wire element. ductively connected to an axially inner ferrule on one of said pair of inner casings; References Cited (e) an electrically insulating heat .transfer medium 15 UNITED STATES PATENTS inside said outer casing filling the space thereof not occupied by said pair of inner casings and said ad- 3418614 12/ $62 Kruegr 337-162 XR (f) a plurality of separate fusible elements em e ed 2,911,502 11/1959 Edsau 337 186 XR in said arc-quenching filler in said pair of inner 20 casings and in said arc-quenching ller in said additional inner casing and conductively interconnect- BERNARD A' GILHEANY Pnmary Exammer ing the ferrules 0n said pair of inner casings and D. M. MORGAN, Assistant Examiner conductively interconnecting the ferrules on said additional inner casing, the fusible elements con- 25 U.S. Cl. X.R.

tained in different of said casings having points of 337-186, 229 

